This invention relates to a security element for protecting objects of value, the security element having a thermochromic layer. The invention relates further to an object of value, a security document and a security paper with such a security element and to a foil material for producing such a security element.
It has been known for some time to use thermochromic materials for protecting documents of value. For example, DT 22 12 350 describes a security thread made of transparent plastic which has cavities. Said cavities contain a liquid-crystalline material which shows a reversible color change upon an increase or decrease of temperature.
EP 0 608 078 B1 likewise discloses a security thread with thermochromic properties. In this case a plastic material is provided with a print or with characters which are formed by partly demetalizing a metal layer. Said print or negative characters bear a thermochromic coating which is colored at normal temperature. When heated the thermochromic coating becomes colorless so that the characters therebelow become recognizable. Alternatively, a thermochromic coating can be used which is colorless at normal temperature and becomes colored when heated so that the characters disappear. Said thread is incorporated into the security paper so as to pass directly to the surface in certain areas, so-called “windows.”
Such thermochromic security threads have the disadvantage, however, that they are very narrow and the thermochromic effect is recognizable only in the relatively small window areas, so that the optical effect evoked by the color change of the thermochromic material is very inconspicuous. Such security elements therefore do not offer very high protection from forgery.